This invention relates to a flip-chip type semiconductor device sealing material and a flip-chip type semiconductor device encapsulated therewith.
With the advance of electric equipment toward smaller size, lighter weight and higher performance, the semiconductor mounting technology has changed from the pin mating type to the surface mounting which now becomes the mainstream. One bare chip mounting technology is flip-chip (FC) mounting. The flip-chip mounting is a technique of providing an LSI chip on its circuit pattern-bearing surface with several to several thousands of electrodes, known as bumps, of about 10 to 100 microns high and joining the chip to electrodes on a substrate with a conductive paste or solder. Then the sealing material used for the protection of FC devices must penetrate into gaps of several tens of microns defined by bumps between the substrate and the LSI chip. Conventional liquid epoxy resin compositions used as the underfill material for flip-chip devices are generally composed of a liquid epoxy resin, a curing agent and an inorganic filler. Of these, the most predominant is a composition in which a large amount of inorganic filler is blended in order to provide a matching coefficient of linear expansion with those of semiconductor chips, substrates and bumps for increased reliability.
With respect to stress properties, the flip-chip underfill materials with high loading of filler give rise to no problem. However, they suffer from very low productivity since they have a high viscosity due to the high filler loading so that they may penetrate into the gap between chip and substrate at a very slow rate. There is a desire to overcome this problem.
In conventional liquid epoxy resin compositions used as underfill material, acid anhydrides are often employed as the curing agent. Since the acid anhydrides are hygroscopic, there arises prior to curing a phenomenon that a viscosity increase due to moisture absorption can cause varying infiltration or interrupt infiltration. Also, the acid anhydrides are likely to pick up moisture prior to curing and even after curing, subject to enhanced hydrolysis by virtue of ester bonds, both of which in turn can cause volumetric expansion, increasing the resistance at the interface between solder bumps and leads. This gives rise to a reliability problem.
From the standpoint of reducing the manufacture cost of flip-chip type semiconductor devices, it now becomes the mainstream in the art to seal flip-chip type semiconductor devices with underfill material without cleaning the devices of the flux used in connecting solder bumps. The omission of cleaning means that the flux is left on the substrate and the circuit portion of the silicon chip, giving rise to several problems. The contact of the residual flux with the underfill material can cause voids to form within the underfill material and preclude effective curing of the underfill material. Also separation can often occur at the interface between the flux and the underfill material.
An object of the invention is to provide a sealing material suitable for an underfill material for a flux-uncleaned flip-chip type semiconductor device which maintains a low enough viscosity to ensure interstitial infiltration even when filled with a large amount of inorganic filler. Another object is to provide a flip-chip type semiconductor device encapsulated with the sealing material and having improved reliability.
It has been found that a liquid epoxy resin composition comprising a liquid epoxy resin and an inorganic filler is improved using a curing accelerator of the general formula (1) to be defined below having a solubility of up to 1% by weight in the epoxy resin, a melting point of at least 170xc2x0 C., a mean particle diameter of up to 5 xcexcm, and a maximum particle diameter of up to 20 xcexcm, and an amino group-containing, polyether-modified polysiloxane of the general formula (3) to be defined below. The resulting composition remains highly stable at elevated temperatures and when used for the encapsulation of flip-chip semiconductor devices, can infiltrate through narrow gaps at a low viscosity even with high loading of the inorganic filler, as long as the semiconductor device is heated to elevated temperatures. The composition thus provides a sealing material suitable as an underfill material for flip-chip semiconductor devices, especially when the flux has not been removed.
As described above, acid anhydrides are widely employed as the curing agent in conventional liquid epoxy resin compositions used as underfill material. Since the acid anhydrides are hygroscopic, there arises prior to curing a phenomenon that a viscosity increase due to moisture absorption can diversify the infiltration capability or interrupt infiltration. Also, the acid anhydrides are likely to pick up moisture prior to curing and even after curing, subject to enhanced hydrolysis by virtue of ester bonds, both of which in turn can cause volumetric expansion, increasing the resistance at the interface between solder bumps and leads. This gives rise to a reliability problem. It is thus believed that how to reduce the saturation moisture pickup is a key to reliability improvement. A study is made on a resin composition having epoxy self-condensation or ether bonds which are less liable to hydrolysis than ester bonds. Use of this curing system can minimize the deterioration of properties by humidity, for example, lowering of bonding force and glass transition temperature (Tg) after water absorption. However, the epoxy self-condensation system has a drawback that it tends to be highly viscous because of the resin""s viscosity and thus has an inferior infiltration capability, as compared with the acid anhydride curing system. This drawback can be overcome by using the curing accelerator which is stable at elevated temperatures and keeping the device at elevated temperatures so that the sealing material may become low viscous and have a high infiltration capability. In addition, as opposed to the acid anhydride system which is unstable during storage and must be kept at very low temperatures for storage, the composition of the invention using a potential catalyst is improved in storage stability and its viscosity remains unchanged over 96 hours at room temperature (25xc2x0 C.).
Accordingly, a sealing material in the form of a liquid epoxy resin composition as defined herein for use with flip-chip type semiconductor devices, especially for flux-uncleaned flip-chip type semiconductor devices has improved thin-film infiltration capability and storage stability as an underfill material. A semiconductor device encapsulated with the sealing material remains highly reliable.
Thus the invention provides a sealing material for flip-chip type semiconductor devices in the form of a liquid epoxy resin composition comprising (A) a liquid epoxy resin, (B) an inorganic filler, (C) a curing accelerator having a solubility of up to 1% by weight in the epoxy resin, a melting point of at least 170xc2x0 C., a mean particle diameter of up to 5 xcexcm, and a maximum particle diameter of up to 20 xcexcm, and (D) an amino group-containing, polyether-modified polysiloxane. The curing accelerator (C) is represented by the following general formula (1): 
wherein R1 and R2 each are hydrogen (xe2x80x94H), methyl (xe2x80x94CH3), ethyl (xe2x80x94C2H5), hydroxymethyl (xe2x80x94CH2OH) or phenyl (xe2x80x94C6H5), R3 is methyl (xe2x80x94CH3), ethyl (xe2x80x94C2H5), phenyl (xe2x80x94C6H5) or allyl (xe2x80x94CH2CHxe2x95x90CH2), and R4 is hydrogen (xe2x80x94H) or a group of the following formula (2). 
The amino group-containing, polyether-modified polysiloxane (D) is represented by the following general formula (3): 
wherein R5 is a monovalent hydrocarbon group of 1 to 6 carbon atoms, R6 is a monovalent hydrocarbon group of 1 to 6 carbon atoms or monovalent organic nitrogenous group having at least one NH group, R7 is a divalent hydrocarbon group of 1 to 10 carbon atoms or divalent organic nitrogenous group having at least one NH group, the compound of formula (3) containing at least one NH group in a molecule, a is an integer of 10 to 200, b is an integer of 1 to 10, c is an integer of 0 to 10, and d is equal to 0 or 1, and Y is a group of the following general formula (4): 
wherein R8 is a monovalent hydrocarbon group of 1 to 6 carbon atoms, e is an integer of 0 to 25, f is an integer of 5 to 50, and e and f satisfy e/fxe2x89xa61 and e+f=10 to 50.
Also contemplated herein is a flip-chip type semiconductor device sealed with the sealing material.